1. Field of the Invention
This invention relates to a resin film for use in wrapping, such as stretch wrapping of foods or pallet stretch wrapping, or in bag formation and a process for producing the same.
2. Discussion of the Background
In the field of stretch wrapping or prepackaging of fresh foods, such as vegetables, fruits, fishes, and meats, and cooked foods, either with or without a plastic tray, films made of polyvinyl chloride-based resins are conventionally used. Recently, however, the use of ethylene-based resins, such as low-density polyethylene and ethylene-vinyl acetate copolymer, as a substitute for the polyvinyl chloride films has drawn much interest for safety and hygiene reasons.
However, films of known low-density polyethylene-based resins, such as low-density polyethylene or ethylene-1-butene copolymer, tend to be difficult to use as stretch wrapping because they are had and less stretchable. Forced stretching of these films results in non-uniform stretching or breaking of the film, or causes deformation or breakage of the food tray to which the film is applied causing the film to wrinkle. This fails to provide sufficient tension for achieving commercially valuable wrapping which must protect the food from loss of freshness and avoid contamination.
While a film of an ethylene-vinyl acetate copolymer, having appropriate properties, such as vinyl acetate content and melt flow rate, would solve above-described problems associated with low-density polyethylene films, these ethylene-vinyl acetate copolymer films, when used in stretch wrapping of sharp-edged foods or trays, tear easily at the sharp edge. Additionally, any small initial tear of the film which occurs during delivery of the wrapped articles tends to run or spread into a big tear, eventually leading to unwrapping.
Use of a laminate film composed of the above-mentioned low-density polyethylene film or a linear low-density polyethylene film and an ethylene-vinyl acetate copolymer film as disclosed in JP-B-2-12187 (the term "JP-B" as used herein means an "examined Japanese patent publication") would also solve the problems of hardness and non-stretchability, but still leads to a film which tears easily during wrapping or delivery. This is especially true when the film is of a reduced thickness, as is now preferred for reasons of conservation of raw materials and reduction of packaging costs.
In order to overcome the disadvantages associated with conventional stretch-wrapping films, a laminate film has been proposed. The laminate film, composed of an intermediate layer comprising a specific 1-butene polymer having laminated on both sides thereof a layer mainly comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 5 to 25% by weight, is obtained by blown-film extrusion at a blow-up ratio of from 3 to 7. This film seeks to provide a thin stretch-wrapping film which is excellent in stretchability, softness and particularly tear strength as well as moderate slip, self-adhesion, sufficient heat-sealability, and transparency, as disclosed in JP-A-61-89040 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), U.S. Pat. No. 4,713,282, and EP-B-198091.
Nevertheless, even the above-described stretch-wrapping film does not always satisfy market demands for wrapping, such as maintaining a tight fit to the shape of an article without wrinkling and recovering from deformation caused during delivery or display without wrinkling.
A wrapping film which exhibits excellent deformation recovery has been proposed as disclosed in JP-B-2-14898, U.S. Pat. No. 4,619,859 or EP-B-137454 and put to practical use.
That film is composed of at least four layers:
1) at least one base layer comprising a mixed composition selected from (A)+(B)+(C), (A)+(B), and (B)+(C); where PA1 2) an H layer comprising a polymer selected from (C); and PA1 3) a surface layer (S layer) comprising at le one polymer selected from (A), (B), and crystalline 1,2-polybutadiene, PA1 (1) the film has a thickness ranging from 6 to 100 .mu.m; PA1 (2) the film has a birefringence .DELTA.n.sub.A ranging from 0.6.times.10.sup.-3 to 8.times.10.sup.-3, the birefringence being defined by equation: EQU .DELTA.n.sub.A =.vertline.n.sub.MD -n.sub.TD .vertline. PA1 wherein n.sub.MD is a refractive index in the machine direction; and n.sub.TD is a refractive index in the transverse direction; PA1 (b 3) when the film is allowed to stand in a silicone oil bath at 140.degree. C. for 3 minutes, the ratio of percent heat shrinkage in the machine direction S.sub.MD to percent heat shrinkage in the transverse direction S.sub.TD (S.sub.MD /S.sub.TD ; hereinafter referred to as heat shrinkage ratio) ranges from 0.65 to 1.50; and PA1 (4) the film has a strain recovery of at least 18 mm as measured by a test method comprising clamping a circular film specimen having a diameter of 100 mm, indenting the center of the specimen with a hemispherical indenter of 20 mm in diameter to cause deformation, and measuring the maximum strain (indentation) which disappears completely upon removal of the indenter.
(A) is at least one ethylene polymer selected from low-density polyethylene, a copolymer of ethylene with a monomer selected from a vinyl ester monomer, an aliphatic unsaturated monocarboxylic acid, and an alkyl ester of an aliphatic unsaturated monocarboxylic acid, and derivatives of the copolymer; PA2 (B) is a soft elastomer having a Vicat softening point of 60.degree. C. or lower; PA2 (C) is at least one polymer selected from
crystalline polypropylene and crystalline poly-1-butene;
such that at least one base layer is adjacent to the H layer.
The above-described multi-layer film is produced by co-extrusion of the layers through a layered annular die, followed by quenching of the extruded tubular film by a water cooling ring. The tubular film is then passed through two pairs of delivery nip rolls and draw nip rolls while heating to 37.degree. C. with hot air. During this heating process, air is blown into the tube to inflate the tube continuously by means of a rectifying contact guide, thereby stretching the film about 3.3 to 3.6 times in the longitudinal direction and about 3.2 to 3.4 times in the transverse direction. The end region of stretching is cooled by directing cold air of 18.degree. C. from an air ring onto the stretched tube, and the stretched film is folded flat with a deflator and drawn up with nip rolls. The stretched film is then subjected to heat setting with hot air of 40.degree. C. for a few seconds. The heat-set film is cut into two sheets of film by slitting its edges in the longitudinal direction, and each film sheet is respectively wound up under constant tension to obtain the desired film having excellent shrinkability and strength.
Because the above-mentioned wrapping film is an oriented film obtained by cold stretching, when it is applied to a tray, such as a food tray, the tray tends to deform when the wrapping film is heat sealed.
Additionally, JP-B-2-12187 discloses a three-layered stretch-wrapping film composed of a linear low-density polyethylene layer having laminated on both sides thereof an ethylene-vinyl acetate copolymer film which is obtained by blown-film extrusion at a blow-up ratio of 3 to 6. However, the film still has deformation (strain) recovery which is inferior to polyvinyl chloride films.